The present invention relates to regenerating an NOx-storage catalytic convertor of an internal combustion engine.
To clean the exhaust of an engine having internal combustion of a fuel, the nitrogen oxide produced during combustion must be reduced. In conventional engines controlled to an average xcex of 1, this can be achieved with good results by a 3-way catalytic convertor. However, there is presently no such established emission control process in internal combustion engines operated at xcex values greater than 1, such as, e.g., lean mix engines, direct-injection spark ignition engines, and diesel engines. In such types of engines, zeolite catalytic convertors (also referred to later as xe2x80x9clean-mix catalytic convertersxe2x80x9d) and NOx-storage catalytic convertors are presently used as methods for treating exhaust gases. The zeolite catalytic convertors are thermally inactivated, which is why they cannot be used in engines for vehicles that must demonstrate a service life in the registration process. Furthermore, these catalytic converters can only use the hydrocarbons in the exhaust for reducing nitrogen oxides, so that only relatively low conversions of nitrogen oxide are attained. These often amount only to 15%, if one disregards the partial reduction of nitrogen oxides to dinitrogen monoxide. The inadequate CO and HC conversions is also a disadvantage of these catalytic convertors, if they have no precious metal. NOx-storage catalytic converters are more promising than the above-mentioned zeolite catalytic converters, since the former use both the hydrocarbons, as well as the hydrogen and CO, in the exhaust as reducing agents. Basically, these are 3-way catalytic convertors having a component for storing NOx. However, the NOx store or storage element becomes clogged with NOx after extended lean engine phases, and is, thus, no longer effective. Therefore, in the case of the NOx-storage catalytic converters, it is necessary to periodically remove the stored NOx from the store, i.e., to reduce the stored NOx.
EP 0 540 280 describes treating exhaust using an exhaust treatment system including a means for storing and releasing NOx, the nitrogen oxides being temporarily stored during lean engine operation and thermally released again by heating the introduced exhaust gases. The released nitrogen oxides are then decomposed under oxidizing conditions by a catalytic converter which decomposes NOx. In particular, the NOx-decomposing catalytic convertor can include a 3-way catalytic converter and/or a zeolite catalytic converter, which is operated at a xcex less than or equal to one. It is particularly disadvantageous that this catalytic converter is not sufficiently thermally resistant; and in order to prevent damage, as typically occurs in such catalytic converters under high loads and exhaust temperatures at xcex=1, an exhaust-gas switching operation requiring appropriate servo and control units for its operation, is necessary. In addition, the problem of durability is not solved with these parts. Furthermore, the question of operating temperature remains unanswered in the parts of the exhaust treatment system having no exhaust gas flowing through them during the stoichiometric engine operation phases; or in the reverse case, in the parts having no exhaust gas flowing through them during lean operation. In this case, the light-off temperature range of the 3-way catalytic converters is particularly problematic, because dinitrogen monoxide is increasingly formed in this phase through partial reduction of the nitrogen oxides from the engine. Should this range be passed through again and again by periodically cooling of the 3-way catalytic converter, one must expect excessive production of dinitrogen monoxide, which is undesirable because of the greenhouse relevance of this gas.
EP 0 562 805, herein incorporated by reference, describes an exhaust treatment system of an internal combustion engine, in which the exhaust system has two lean NOx catalytic converters that are arranged in parallel and have exhaust gas alternately flowing through them. In addition, the known arrangement includes a device for changing the space velocity of the exhaust, in order to be able to set an optimum space velocity of the exhaust. Furthermore, the exhaust system has a means of injecting HC directly into the exhaust flue. The service life is also questionable in this case, since zeolite catalytic converters are not thermally resistant, and in particular, do not tolerate rich or stoichiometric exhaust. The service life of the device is also problematic with regard to changing the space velocity of the exhaust switching device. Even if better adapting the space velocities of the exhaust produces an NOx conversion higher than in typical zeolite catalytic converters, the catalytic converter according to EP 0 562 805 does not reach the magnitude of over 90% required for complying with the new exhaust emission standards. Moreover, the problems of dinitrogen monoxide formation and the HC and CO conversion of these catalytic converters being too low, remain unsolved.
EP 0 580 389 illustrates a process for treating the exhaust of leanly operated engines, which are equipped with an NOx absorber having an alkali, alkaline-earth, or rare-earth metal base, a 3-way catalytic converter arranged downstream, as well as sensors for detecting the load and the exhaust temperature. In this context, the information from the sensors is used to define the range in which the NOx absorber is able to store nitrogen oxides. The catalytic converter is regenerated by enrichment for a predefined period. A disadvantage of this known device is the separation of the absorber and the 3-way catalytic converter, since the nitrogen oxides predominantly generated by the engine must initially be oxidized to NO2 in order to be able to be stored in the absorber.
EP 0 560 991 describes a system for treating exhaust of an internal combustion engine, in which the absorber and the catalytic converter are contained in a housing. The nitrogen oxides are stored when the engine is operated leanly, i.e., when the exhaust is lean, and are released when the oxygen concentration in the exhaust is lowered to rich or stoichiometric xcex values, so that the released NOx is reduced by the unburned hydrocarbons and the CO of the exhaust. Switching over from lean to rich or stoichiometric operation is typically accompanied by sudden changes in torque, which are only desirable to the vehicle driver, when they occur during an acceleration phase. These sudden changes in torque are extremely undesirable, if they occur during a constant operation phase. Since the NOx store is normally emptied during constant operation phases, it is attempted to reduce these sudden changes in torque by adjusting the ignition timing simultaneously to the enrichment.
Furthermore, the presently known NOx-storage catalytic converters are inactivated by sulfur-containing fuel. The material absorbing NOx in the NOx-storage catalytic converter, especially BaO or BaCO3, reacts with the SO2, which is present in the exhaust and is oxidized to SO3 at the platinum present in the catalytic converter, to form thermally stable sulfates that can be decomposed at a temperature lying above the decomposition temperature of the nitrates formed from the store material and the NO2. In order to decompose these sulfates, a sulfate regeneration program is therefore executed from time to time, as a function of the sulfur content of the fuel being used; the temperature being increased to approximately 600-700xc2x0 C. by enriching the exhaust, so that the sulfates decompose. However, the disadvantage of the enrichment is that this normally correlates to an increased power output of the engine, so that carrying out desulfation finally causes the vehicle to accelerate unintentionally.
The present invention provides a device and/or a method for treating exhaust of an internal combustion engine, which reduces the NOx concentration of the exhaust and/or the sulfate content of the NOx store without effecting a sudden change in torque or an increased power output.
The present invention provides a method for Nox and/or Sox regeneration of an Nox-storage catalytic converter, which is arranged in an exhaust treatment system of an internal combustion engine having more than one cylinder; a mass flux of reducing agents (HC, CO, H2) being increased in the exhaust in order to regenerate the NOx-storage catalytic converter, wherein by means of a control unit, a part of the cylinders is operated under lean conditions (xcex greater than 1) and another part of the cylinders is operated under rich conditions (xcex less than 1) (cylinder-selective detuning); however, the average over all of the cylinders is xcexxe2x89xa71.
Another embodiment of the present invention provides a method wherein a part of the cylinders is operated during the regeneration at xcexxe2x89xa60.95, and more preferably at xcexxe2x89xa60.85. Another embodiment of the present invention provides a method as recited in either embodiment above wherein the cylinders are selectively detuned during a constant operating phase without load alteration.
Another embodiment of the present invention provides a method as recited in any of the embodiments above, wherein half of, or a number close to half of, the cylinders is enriched.
Another embodiment of the present invention provides a method as recited in any of the embodiments above, wherein the control unit selectively detunes the cylinders at idle, in deceleration, and/or in response to an engine load xe2x89xa625% of the maximum engine load.
Another embodiment of the present invention provides a device implementing any of the methods of the above-described embodiments.